1. Field of the Invention
This invention relates to improved compositions of etching solutions containing soluble surfactant additives. These additives are capable of maintaining reduced surface tensions for improved substrate wetting and are useful in integrated circuit manufacture. Specifically, this invention relates to the utilization of perfluronated alkylsulfonates and/or perfluronated amphoteric surfactants in polar solvent mixtures as additives for suppression of surface tension.
2. Prior Art
As integrated circuit component dimensions become smaller, physical wetting by etchant solutions on substrate surfaces becomes more difficult. This is especially important for buffered oxide ammonium fluoride/hydrofluoric acid etchant solutions used in silicon dioxide etching, since these solutions exhibit extremely high surface tension values of 85 to 90 dyne/cm at typical etching temperatures. Because of the relatively low surface energies of the masking materials used, the photoresist topography, the types of impurities present, and the residual contaminants from other processing steps, it is difficult to properly wet the substrate, which results in non-uniform etching and poor line reproduction.
To overcome these problems, much of the integrated circuit industry has used one of at least two known techniques. The first involves a predip in an aqueous surfactant solution before placing the substrates in the etchant while the second utilizes direct addition of the surfactant to the etchant solution. However, as the industry progresses towards the use of more particulate free systems in which refiltration tanks are employed, and at the same time requires more precisely etched lines with less variation, existing methods have several shortcomings. With the use of predipping, two tanks are required instead of one, which adds an additional processing step. In addition, since wafers are being presoaked, there is a tendency to carry material over from the predip tank to the etchant tank, thus changing the etchant performance and shortening its useful life. More importantly, a major disadvantage associated with both of these methods is that, as experience has indicated most surfactants commonly used by the industry are insoluble in ammonium fluoride/hydrofluoric acid oxide etchant solutions, which causes plating out of the surfactant onto substrate surfaces and plugs the filters (which are of the order of 0.2 micron) commonly used in recirculating etch baths and thus yields etchant solutions with little or no surfactant present. Also, the surfactants used may contain metal ion impurities which can be detrimental to integrated circuit performance or lose activity due to surfactant degradation caused by the presence of hydrofluoric acid in the etchants. Several etchants manufactureres have attempted to incorporate a surfactant in their oxide etchants solutions. However, analysis of these materials shows little, if any, surfactant is present as indicated by surface tension measurements. It is thus apparent that a need exists for an improved, effective silicon dioxide etchant solution with lower surface tension wherein the surfactant maintains surface activity in ammonium fluoride/hydrofluoric acid solutions after filtration through 0.2 micron absolute filters and is essentially free of metal ion.